For example, in the case of a rod-shaped sushi (bo-zushi), vinegared rice is shaped, and the shaped rice section is topped with a sushi material or materials such as boned and prepared red sea bream, sockeye salmon or tuna together with a thin slice of kombu kelp (shiroita kombu) cooked in a broth seasoned with vinegar, sugar and salt, and resultant is shaped into various shapes such as the rod-shape, and the shaped sushi product is wrapped with a bamboo sheath and placed in a packing bag made of a plastic film and vacuum-packed, the vacuum packed sushi product is frozen with a brine such as ethanol or the (vacuum-packed) shaped sushi product is frozen, and the frozen shaped sushi product is placed in a packing bag made of a plastic film and vacuum-packed to thereby prepare a packed frozen sushi product.
Such a packed frozen sushi product by means of brine freezing is prepared by placing a sushi product in a plastic packing bag, vacuum-packing the sushi product, and brine freezing the vacuum-packed sushi product. Accordingly, no attachment of bacteria to the sushi product or absorption of odor from the coolant gas in the sushi product in a freezing room is caused, and therefore, the packed frozen sushi product is hygienically superior to, for example, a packed frozen sushi product prepared by vacuum-packing a sushi product posterior to freezing of the sushi product by slow freezing such as air-blast freezing, nitrogen or carbon dioxide freezing. In particular, such a packed frozen sushi product frozen by brine freezing can be frozen-stored stably, for example, over a long period of 3 months or longer when stored at a storage temperature of −18° C., or can be stored for 6 months when stored at a storage temperature of −50° C.
However, a packed frozen sushi product which has been frozen-stored need to be thawed before being served at a meal. Methods for thawing a packed frozen sushi product include natural thawing, running water thawing, steam thawing, hot water thawing and microwave thawing. Of these, microwave thawing is quite suitable for thawing of a packed frozen sushi product or the like due to its extremely short thawing time and high thawing yield. For example, in the case of above mentioned rod-shaped sushi product, natural thawing at room temperature requires a thawing time of five hours, whereas microwave thawing enables thawing in a period of 2 to 5 minutes. Accordingly, microwave thawing is capable of instantly responding to demands and thus convenient as compared with natural thawing at room temperature.
However, in the case of microwave thawing, calorific values derived from microwave absorption are different between a sushi material or materials and a rice section, and these are also different in quantity. Further, because a surface portion is intensively heated as compared with a center portion, a portion of the sushi material or materials is likely to be heated to an undesirably high temperature as compared with the rice section. The sushi material or materials are thereby brought into the so-called cooked condition, and the tastes thereof are impaired. Therefore, microwave thawing has not been practically used. In a case of, for example, a boxed sushi product or a rod-shaped sushi product, in general, a rice section is preferably lukewarm, for example, the rice section preferably has temperatures of 15 to 25° C., more preferably 20 to 25° C. although this is dependent upon the type of the sushi product, whereas a sushi material or materials, for example, conger eel of eel is preferred to have roughly the same temperatures as the rice section, for example, temperature of 10 to 25° C., more preferably 10 to 20° C. although this is dependent upon the type of the sushi material or materials. However, when a packed frozen sushi product is thawed in a microwave oven, the material or materials are heated to relatively high temperatures to impair the taste of the sushi product. Accordingly, it is difficult to effect desired thawing by means of a microwave oven.
Under such circumstances, with a view to solving the problem that in thawing a packed frozen sushi product by means of microwave heating using a microwave oven or the like, a sushi material or materials of a sushi product are liable to be heated to relatively high temperatures to thereby impair taste of the sushi product, the present inventors made a proposal as follows. To wrap a sushi product topped with a sushi material or materials with a plastic wrapping film, place the wrapped sushi product in a plastic packing bag for frozen foods, vacuum the packing bag to vacuum-pack the sushi product, freeze the vacuum-packed sushi product en bloc by brine-freezing in a unified form to prepare a so-called packed frozen sushi product suitable for freeze-storage, and in thawing the thus prepared packed frozen sushi product by means of a microwave oven, terminate heating by the microwave oven in a relatively short time so as not to raise temperatures of the sushi product to undesirably high temperatures, and using water vapor emanating from a heated rice section and prevailing in the plastic wrapping film package during the microwave heating and after removal of the sushi product from the microwave oven, heat the sushi product by steaming with the water vapor generated in the plastic wrapping film package and the residual heat of the rice section to bring a temperature of a portion where the sushi material or materials and the rice section are in contact with each other to 15° C. or higher (see Japanese Patent publication No. 3172677, and the specification of U.S. Pat. No. 5,861,184).
For preparing such a packed frozen sushi product, however, it is necessary to tightly wrap a sushi product with a plastic wrapping film, place the wrapped sushi product in a plastic packing bag for frozen foods, and vacuum and hermetically seal the packing bag to vacuum-pack the sushi product. This procedure should be performed carefully, and such a packed frozen sushi product is difficult to prepare on a large scale. Accordingly, production of such a packed frozen sushi product to meet customers' demands involves many manual labors and much time.
It is an object of the present invention to provide a packed sushi product which is capable of solving the problems inherent in such a conventional frozen sushi product wrapped with a plastic film and which is adapted for thawing by means of a microwave oven.